When you go to a country with a weak currency you should never change your dollars, euros, yens etc. at home. You should do the exchange on the spot. It is because countries with soft currencies are more eager to trade their shitty money than hard currency countries. As the risk is much higher for let's say a German bank to hold Turkish lira which devalue almost on a daily base than for Turkish banks to stock euros and liras. Liras are reprinted in Turkey not in Germany.

In the past especially Eastern bloc countries had a flourishing black market to exchange currencies. When you made a trip to Czechoslovakia, you had at least three different rates for German marks. The German one, the official Czech one and the Czech street rate. To make things more complicated there were several black market rates. When you tried to exchange your German marks on the street the dealers always used the same trick. As an example the rate was 15 Czech crowns to one German mark. So you gave two hundred marks and the guy gave you a wad of Czech banknotes. You counted them and it turned out that hundred crowns were missing. So you gave the packet back to your seller. And he added the missing banknote while removing five to ten of the hundred crown notes below without you noticing it. Who would ever go through a pile again to recount it?

An acquaintance of mine watched this procedure for a couple of times. One day he met someone who offered him 21 crowns for one mark. He accepted to exchange two hundred marks. When he counted the money there were only 40 hundred crown banknotes. He said to the dealer: Ok, everything is fine. Who wasn't happy of course and started shouting and threatened to inform the police. Which just raised a smile on the face of my acquaintance. He left and never succeeded to spend all his cheaply obtained money. There simply was nothing useful to buy for it except bread, toilet paper and beer.

P.S. I am not sure if the amounts, exchange rates or even the country is correct but I think you can get the gist anyway.